The largest and best supported Jaguar cars enthusiast site on the Internet!

Serving Enthusiasts since 1993
The Jag-lovers Web

Currently with 3,166 members

Jaguar XJ-S Bulb Failure Sensor

Jaguar XJ-S

Bulb Failure Sensors

The Jaguar XJ-S has bulb failure sensors that detect when a lighting circuit is not drawing as much current as it should and turns on a light on the dash telling the driver that a light bulb is burned out.  There are actually two different types of sensors:  a single sensor in the console that monitors the brake lights, and four sensors that monitor the parking lights, one for each corner of the car.  This is what the latter type look like:

This particular one is for the right rear of Kirby Palm's '83, and it mounts along the right side of the trunk opening.

Supposedly, some earlier or European models had only three sensors instead of four, using a single sensor to monitor all the parking lights on the front of the car.

These sensors are thermal in nature.  Inside each sensor is a resistance that heats up due to the current flowing through on its way to the lights.  If there is enough current, the heater gets hot enough to cause a bimetal strip to open a pair of contacts to extinguish the warning light on the dash.  Of course, this heating takes time; that's why the warning light is on for about the first twenty seconds after you turn on the lights.

There is an adjustment on these sensors; a tiny screw that moves the contact that the bimetal strip closes on.  It's visible in this picture at the upper left, but the screw head has been dyked off on this one.  It can still be turned with a pair of pliers.  Using this adjuster, the sensors can be tuned to respond at exactly the right current level -- but it appears Jaguar didn't want you fiddling with it, and instead sells these units pre-adjusted.

Since they work by heating, these sensors take power.  In fact, these bulb failure sensors seem to soak up about two volts of the power to the lights.  So, the lights that should be operating at full system voltage (13.8 - 14.4V) are actually operating at somewhere around 11-12V.  This makes the lights look anemic, since the brilliance of incandescent lights is very sensitive to voltage.

Palm is working on a replacement sensor design that uses a magnetically-operated switch.  The current to the lights goes through a coil, and the magnetism generated by the current through the coil opens the contacts in the switch to extinguish the warning light on the dash.  By wrapping the coil with thick wire, the voltage loss can be reduced to insignificance and the lights will operate at full brightness.  Since the switch will operate instantly, the warning light won't be on for twenty seconds any more.

A suitable magnetic switch has been found.  It is made by GC, part number 35-752, "Normally Open/Normally Closed Magnetic Reed Switch", available at better electronics supply stores.  The package includes the switch itself, a spool suitable for winding the coil, and a permanent magnet.  Here's the switch and spool:

No need to show the magnet, since it isn't used in this project.

Yeah, the thing is small.  It's sensitive, too; you get a magnet anywhere near this thing and it switches.  The contacts are rated at 0.5A, which is enough to operate that warning light.

I used a green background to make the white spool show up in the picture, but the glass tube surrounding the switch also has a green tint.

The plan is to insert the switch into the center of that spool and then wrap 16 gauge wire around just until the switch opens with the lighting current, and then perhaps one or two more turns.  There is no convenient way to adjust such a sensor other than by varying the number of turns.

The project isn't done yet; once done, this page will be updated with more pictures.


Please help support the move to the new site, and DONATE what you can.
A big Thank You to those who have donated already!



Go to our Homepage
Improve your Jag-lovers experience with the Mozilla FireFox Browser!

  View the latest posts from our Forums via an RSS Feed!

Jag-loversTM Ltd / JagWEBTM 1993 - 2024
All rights reserved. Jag-lovers is supported by JagWEBTM
For Terms of Use and General Rules see our Disclaimer
Use of the Jag-lovers logo or trademark name on sites other than Jag-lovers itself in a manner implying endorsement of commercial activities whatsoever is prohibited. Sections of this Web Site may publish members and visitors comments, opinion and photographs/images - Jag-lovers Ltd does not assume or have any responsibility or any liability for members comments or opinions, nor does it claim ownership or copyright of any material that belongs to the original poster including images. The word 'Jaguar' and the leaping cat device, whether used separately or in combination, are registered trademarks and are the property of Jaguar Cars, England. Some images may also be Jaguar Cars. Mirroring or downloading of this site or the publication of material or any extracts therefrom in original or altered form from these pages onto other sites (including reproduction by any other Jaguar enthusiast sites) without express permission violates Jag-lovers Ltd copyright and is prohibited
Go to our Homepage
Your Browser is: CCBot/2.0 (, IP Address logged as on 26th Feb 2024 07:03:22